Preheat your oven to 350°F (175°C) and prepare two 9-inch round cake pans by greasing or lining them with parchment paper.

In a medium saucepan, combine the unsalted butter, water, and unsweetened dark cocoa powder over medium heat, stirring until melted and smooth.

In a large mixing bowl, whisk together the all-purpose flour, light brown sugar, baking soda, fine grain sea salt, and espresso powder, if using.

Gently fold the cooled chocolate mixture into the bowl of dry ingredients using a spatula until just combined.

Add the buttermilk, eggs, and vanilla extract to the batter, mixing until smooth and fully combined.

Divide the batter evenly between the prepared cake pans and bake for 30-35 minutes until a toothpick comes out clean.

Allow the cakes to cool in the pans for about 10 minutes, then transfer to wire racks to cool completely.

Whip heavy cream in a mixing bowl until stiff peaks form, then fold in the semisweet chocolate chips.

In another bowl, blend cream cheese, white chocolate, confectioners' sugar, and softened butter until smooth.

Once the cake layers are cool, place one layer on a serving platter and spread half of the chocolate mousse over it, followed by half of the cream cheese mixture. Top with the second layer and repeat.

Melt the semisweet chocolate chips, heavy cream, and sunflower oil in a saucepan over low heat, stirring until smooth, and pour over the assembled cake.

Decorate the cake with additional frosting, chocolate truffles, and rolled cookie wafers as desired.